The present invention relates to a method for determining the alignment or misalignment of the axes of a coordinate measuring machine.
In order for coordinate measuring machines (CMM) to operate properly, their axes must be aligned perpendicularly with respect to each other and their position transducers must be properly positioned with respect to the axes. Heretofore, if a CMM has been in need of adjustment, extremely time-consuming procedures have been used to determine and make the necessary adjustments. For example, one prior method for determining axis misalignment is as follows:
(a) Positioning a granite cube on the CMM table with a first side aligned with the CMM X axis and then measuring the variation in the CMM-generated Y coordinate as the CMM probe is moved over the first side, then adjusting the cube position until no Y variation is produced.
(b) Move the CMM probe over a second side (perpendicular to the X axis) and measure the variation in the CMM-generated X coordinate. The ratio of the X coordinate variation to the Y coordinate variation is a measure of the misalignment between the CMM X and Y axes.
(c) Measure Y and Z axis misalignment by repeating steps (a) and (b), using appropriate sides of the granite block and substituting Y for X and Z for Y in steps (a) and (b).
(d) Measure X and Z misalignment by repeating steps (a) and (b), using another pair of sides and substituting X for X and Z for Y.
In addition to being time-consuming, this granite square method is subject to errors caused by imprecise positioning of the granite square on the CMM table.
Another time-consuming method is used to measure axis scale errors and involves the use of a laser and the following steps:
(a) A reflector for a laser interferometer is attached to the CMM in place of the CMM probe.
(b) The Y and Z axes of the CMM are locked so that only movement along the X axis is allowed.
(c) A laser interferometer is alinged so that its beam travels parallel to the X axis and strikes the reflector.
(d) The reflector is then moved along the X axis of the CMM and CMM-generated X axis readings and the interferometer readings are obtained. From these readings the scale error in the CMM X axis can be determined.
(e) Steps a-d are then repeated for the Y and Z axes.
Also known are CMM inspection procedures which involve the use of artifacts such as the barbell and the Bryan Gauge. However, the data generated by the CMM during these procedures has heretofore been utilized only on a pass-fail basis, and has not been used to determine exactly what CMM adjustments are needed. In other words, if use of the above artifacts indicated that adjustment was required, then the previously described granite block or laser interferometer procedures would have to be used in making the needed adjustments.
The above-described time-consuming methods are inefficient because they require frequent manual positioning of hardware devices such as the granite block and the laser interferometer. Then all the data generated by these procedures has to be recorded and manipulated, after which further adjustments and checking may be needed.